In the transmission of color television signals by the NTSC and PAL systems chrominance information is conveyed by modualtion of a subcarrier so as to produce sidebands lying in the upper portion of the spectrum used for the transmission of luminance information. It has been found that diagonal black and white patterns that are fine enough to produce frequencies within the portion of the spectrum occupied by the subcarrier can cause a receiver to introduce colors into the image. It has therefore been proposed that the decoding circuits be modified when a purely black and white image is present, but there remains the difficulty of detecting the existence of this condition.